1. Field of the Invention
The invention relates to a process for the preparation of chloromethane from methanol and hydrogen chloride contaminated with silicon compounds.
2. Background Art
For the preparation of chloromethane from methanol and hydrogen chloride which originates from the methylchlorosilane hydrolysis, silicon compounds are introduced into the chloromethane reactor. The distillate of the chloromethane reactors therefore contains, as an undesired impurity, methylchlorosilanes, methoxymethylsilanes, hydrolysis and condensation products (polymethylsiloxanes) thereof (referred to collectively as Si compounds below) and hydrocarbons formed as a by product in the methylchlorosilane synthesis (Müller-Rochow). Cyclic polydimethylsiloxanes such as hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), in particular D4, are the main constituent of the Si compounds. These Si compounds cannot be adequately separated from the chloromethane gas by condensation. A portion of the Si compounds condenses in the water scrubber and is fed together with the outflowing MeOH/water mixture to the distillation for methanol recovery. The other portion of the Si compounds is stripped out from the water scrubber together with the MeCl and then converted into siloxanyl sulfates in the drying of the MeCl in the sulfuric acid scrubber. The content of siloxanyl sulfates complicates the disposal or the recycling of the sulfuric acid since silica-like solids which are difficult to handle are formed. The Si compounds fed to the distillative methanol recovery with the methanol/hydrochloric acid mixture leave the column partly together with dilute hydrochloric acid in the form of siloxanols. The other portion of the Si compounds is transported back into the MeCl reactor together with the methanol recovered as the overhead product.
Siloxane producers lose many tonnes per year of Si compounds via the wastewater and especially via the sulfuric acid. These Si compounds cause problems in the handling of the sulfuric acid used for drying and additionally contaminate the environment with poorly biodegradable organosilicon components.
The 1st stage of the hydrolysis of dimethyldichlorosilane to polydimethylsiloxanes is frequently carried out with a substantially stoichiometric amount of water, i.e. with 1 mol of water per 1 mol of Me2SiCl2. In this procedure, the chlorine present in the organochlorosilane is not obtained in the form of hydrochloric acid but in the form of substantially dry hydrogen chloride gas. The energy-consumptive production of hydrogen chloride from hydrochloric acid is then not necessary. Hydrogen chloride gas is required for the preparation of MeCl, which in turn is used for the preparation of methylchlorosilanes from Si.
All hydrolysis processes of organochlorosilanes which give hydrogen chloride gas directly have, however, the disadvantage that the hydrogen chloride obtained is contaminated with silicon compounds and hydrocarbons. The purification of hydrogen chloride which is obtained in plants producing chlorosilanes or siloxanes and contains Si compounds and/or alcohols as an impurity is effected in general by washing the gas mixture with a wash liquid, such as water, hydrochloric acid or sulfuric acid. The purification of hydrogen chloride by distillation is also described. Little has been reported to date about the purification of hydrogen chloride from the hydrolysis process of dimethyldichlorosilane, which contains Si compounds and hydrocarbons.
In an integrated plant for the preparation of α,ω-dihydroxypolydimethylsiloxanes, cyclic polydimethylsiloxanes and MeCl, it is possible to use α,ω-polydimethylsiloxanes as a purification liquid for separating off Si compounds from HCl gas. In this procedure, however, numerous unsolved secondary problems occur owing to the reactivity of the α,ω-dihydroxypolydimethylsiloxanes.
Owing to the insufficient purity of the hydrogen chloride originating from the organochlorosilane hydrolysis, the hydrogen chloride can be converted into MeCl only in liquid-phase processes with methanol. The distinction is made between catalyzed and uncatalyzed liquid-phase processes. The choice of the catalysts for the liquid-phase processes is limited owing to the impurities in the hydrogen chloride. However, catalyzed processes have the advantage of higher space-time yields and higher yields with respect to methanol and HCl. The catalysts frequently used in liquid-phase processes are metal chlorides having Lewis acid properties, such as zinc chloride, iron chloride, bismuth oxychloride, or amines, or quaternary ammonium or phosphonium compounds.
A liquid-phase process for the preparation of MeCl with the use of hydrogen chloride (gas) and methanol is described, for example, in EP 0 428 166 A1. However, the quality of the hydrogen chloride used is not discussed there. Si compounds introduced via the hydrogen chloride must enter the process either in the product MeCl or via the resulting water of reaction, as wastewater depending on the pressure/-temperature conditions prevailing in the MeCl reactor. Where the Si compounds entrained via the hydrochloric acid used or the hydrogen chloride gas remain or how they are treated is not described. In some cases, the processes described in the literature were tested only in laboratory experiments. Problems with Si compounds present in low concentration are not recognized here. The further processing of the MeCl or the distillative recovery of MeOH used in excess is not described.
In the distillative working-up of the mixtures of water, MeOH, HCl and Si compounds inevitably obtained in the MeCl synthesis, the Si compounds frequently polymerize. Blockage occurs in apparatuses and pipelines. The heat transport in heat exchangers may be hindered. The Si compounds remaining in the process wastewater are not biodegradable and cause so-called “persistent COD”. Such Si compounds should be avoided in wastewater for environmental reasons.
MeCl contaminated by Si compounds cannot be used as a raw material for the direct synthesis of organochlorosilanes from MeCl and Si. The separation of Si compounds from MeCl requires additional complicated separation methods, e.g. distillations. If the Si compounds are not removed from the MeCl, they cause problems by formation of silica-like solids in the frequently used treatment with concentrated sulfuric acid for removing dimethyl ether (DME) and for drying.
The unreliability of the process increases as a result of the Si compounds entrained into the MeCl synthesis. A loss of siloxane is observed, which reduces the yield of the process. The environment is polluted.
In the so-called methanolysis processes of methylchlorosilanes, the Si-bonded chlorine is converted directly into MeCl by reaction with methanol/water mixtures. In these processes, too, an MeCl contaminated with Si compounds may be obtained. DE 2521742 A1 describes such a process. The mixture of MeCl, MeOH, HCl and DME leaving the reactor is partly condensed and the noncondensing constituents are washed with methanol cooled to 12° C. The methanol scrubber apparently serves for separating off water, HCl and DME from the MeCl. No information is given regarding the composition of the material streams fed to and removed from the scrubber.
DE 3146526 A1 describes a methanolysis process in which Si-containing MeCl is removed as a vapor mixture from the reactor. A part of the Si compounds present in the MeCl is removed by condensation. Nothing is said about the residual content of Si compounds and the further processing of the MeCl.